Someone with search skills may find threads regarding Kasemann Circular Polarizers (also called Kaesemann). I typed in quotes from B&H Photo Sourcebooks and such, but now am on the road--holidays with relatives--so have no access to the original info...
B+W and Heliopan Kasemann Circular Polarizers and Kasemann Warming Circular Polarizers are arguably the best and most certainly the most expensive. Better humidity and temp sealing, thinner more transmissive foils, less greenish color cast that may result from "standard" polarizing foils. Designed for use with high-end telephoto lenses as well as standard lens sizes.
I also get good results with Pentax Polarizers which you may find on the used market.
Sing Ray offers a quality upgrade over the Cokin polarizers, yet still fit in the Cokin P holders.
I personally use Pentax for small sizes--49mm, 58mm and 67mm. Kasemann for 77mm and 82mm. Oh, I have a low-ball Tiffen 40.5mm for a niche lens too. Will soon replace that one with a Kasemann 40.5mm.